Reading Audi were useless. As soon as they found out my car was out of warranty (by a matter of months) they weren't interested in looking for the TSB, they just wanted to book the car in for 2 hours of chargeable diagnostics to dry the boot out prior to a pressurised water test.
Anyway, if you want a job doing and all that, so I whipped the inner tail lights off that night and I believe to have found the cause of the issue. The seals around the inner tail light clusters are rubberised foam. On the left side tail light (pictured in my OP) the seal had been crushed and it'd been moved away from its seat. I cleaned it all up, heated up the glue on the seal and reset it in the right position before carefully refitting it on the tailgate. No sign of water ingress since then but its early days.
Unfortunately though I've now noticed that one of the LEDs is out on the right side inner light cluster. Gutted!
This thread on Audizine both suggests that LED failure can be caused by the same water ingress issue I experienced, and also that the tail light seals are faulty and are being routinely replaced by Audi as suggested by blackphantom.
Going to try approaching another local Audi dealer armed with this information. Watch this space.
